1. Field of the Invention
This invention relates to binary phase shift keying (BPSK) symbols, and, more specifically, to procedures for BPSK modulation and mapping digital data into BPSK symbols.
2. Related Art
BPSK modulation is a robust modulation scheme that is often used in adaptive coding and modulation systems. It is frequently used for communicating with receivers on the edge of coverage. In BPSK modulation, a BPSK symbol mapper maps digital data into BPSK symbols using a BPSK symbol constellation (also simply referred to as a BPSK constellation). The BPSK symbol constellation is typically represented on a Cartesian coordinate plane in which one of the coordinates represents the in-phase (I) component of the BPSK symbol, and the other of the coordinates represents the quadrature (Q) component of the BPSK symbol.
A quadrature modulator then modulates a carrier signal with the I and Q components of the BPSK symbol. In a quadrature modulator, each component is passed through a pulse-shaping filter, and then up-converted to the transmission frequency using local oscillator signals which are 90° out of phase with one another. The up-converted components are summed, and the ensuing signal is then passed through a power amplifier and transmitted over a wireless interface.
The pulse-shaping filter in a quadrature modulator is typically implemented as a (square) root-raised cosine pulse-shaping filter. For BPSK symbols, the root-raised cosine pulse-shaping filter increases the peak amplitude of the envelope of the signal that is input to the power amplifier. This in turn requires a reduction in the average power of the signal that can be transmitted at the same distortion level, which in turn limits the range of the signal. Alternatively, or in addition, it increases the quiescent power that must be consumed by the power amplifier in order to avoid an increase in the distortion introduced by the power amplifier.
FIG. 1 compares the envelope peaking that is introduced into BPSK, quadrature phase shift keying (QPSK), and 8 phase shift keying (8-PSK) symbol modulation by a root-raised cosine filter having a roll-off factor of 0.25. Curve 102 represents the probability distribution of the envelope peak of a BPSK-modulated signal referenced to average power (expressed in dBc). Curve 104 does so for QPSK-modulated signals, and curve 106 does so for an 8-PSK modulated signals.
These curves illustrate that the envelope peaking that is introduced by the filter into BPSK-modulated symbols substantially exceeds that of the other two modulation schemes. Curve 102, for example, indicates there is about a 1% probability that the envelope peak to average ratio for a BPSK-modulated signal will exceed 4.3 dB, and curve 104 indicates that there is about a 1% probability the envelope peak to average ratio for a QPSK-modulated signal will exceed 3.4 dB. That means that, compared with QPSK symbols, BPSK symbols must be transmitted at 0.9 dB less average power when the fidelity criterion which is applicable is not to clip any more than 1% of the envelope peaks for either modulation scheme. Expressed another way, the average transmit power for BPSK symbols has a relative back-off of 0.9 dB compared to QPSK symbols. That in turn limits the range of BPSK symbols relative to QPSK symbols when transmitted over a wireless interface.